One of today's foremost problems is the disposal of domestic, agricultural and industrial wastes, the protection of the environment to limit air and water pollution to non-damaging levels, and the need to accomplish these goals in an economic manner. It is fair to say that presently this goal has not been reached. As a result, municipalities face almost insurmountable solid waste disposal problems and both the inland and oceanic waters are becoming increasingly polluted, particularly in the vicinity of population centers or concentrations of industry.
The heretofore common practice of dumping solid wastes or using it as landfill becomes less and less feasible as suitable sites are exhausted. Moreover, dumping of solid waste requires extensive land transportation and is therefore costly. As an alternative to dumping it has been proposed to incinerate solid waste. Although solid waste incineration reduces the volume of the waste to be disposed of, it has serious shortcomings. First of all, its operation is normally uneconomical. In addition, it leads to serious air pollution unless the exhaust gases from the incinerator are cleaned by washing them in water. This in turn pollutes the increasingly scarce water. Moreover, solid waste incineration poses serious health hazards because heavy metals in the waste are vaporized. The vapors escape with the exhaust gases and pollute the atmosphere with dangerous heavy metal fumes.
While the solid waste disposal represents to a large extent an economic problem, water pollution, the disposal of sewage and an assured, adequate water supply for domestic, agricultural and industrial use pose an even more serious problem. Aside from economic considerations, the total water supply is limited and not replenishable; that is, water can neither be fabricated nor can the overall supply be increased. The available fresh water supply in any geographic region and worldwide is limited to that available from natural precipitation, the runoff therefrom, and available ground water supplies.
In almost all instances, once water has been used, it becomes polluted water (hereinafter sometimes referred to as "used water") which can no longer be effectively used and which pollutes clean water if it comes in contact therewith. Thus, it is necessary to treat used water at least to such an extent that it no longer harms the water, say sea or river water, into which it is discharged. Preferably, however, used water is cleaned to a sufficient extent to render it again usable.
In the past, it has become a widespread practice to clarify waste waters through an initial, more or less effective preclarification in a biologically activated zone of a clarification plant to reduce the pollutant content of the water. The clarification zones are thought to remove from the water dissolved and suspended organic and inorganic materials in a separation or flocculation process which forms a sludge that can be separated from the water, normally through sedimentation. It employs chemical precipitation agents which are to cause ameliorization, that is, which are to render dissolved substances hydrophobic.
It has been established that this process effects a considerable clarification of the water so long as the micro-organisms of the biologically active zone are not subjected to dangerous toxic substances. The clarification efficiency of the process decreases rapidly when heavy metal contaminations, say from industrial waste, or toxic substances from dissolved agricultural fertilizers and pesticides, etc. are encountered because these contaminants damage the micro-organisms and reduce their activity and effectiveness.
Assuming that the micro-organism attacking contamination levels in the waste water can be kept to tolerable levels, the prior art water clarification processes generally withdraw sludge which normally has a particle or solid content of about 3%. The sludge is then dehydrated in a number of somewhat cumbersome steps, first by concentrating the sludge (which yields highly polluted waste water that is returned to the clarification plant) and thereafter by passing the concentrated sludge through a filter press or a centrifuge where a particle concentration of up to 30 to 40% is achieved. The concentrated sludge can then be disposed at suitable disposal sites. However, since such sites are becoming increasingly scarce, the sludge is frequently incinerated in accordance with a variety of processes.
For example, the sludge may be combined with previously generated ash in the energy inefficient, so-called sludge-ash incineration method which requires substantial amounts of external energy, e.g. fuel to effect the incineration of the sludge removed from the waste water and which recyles and, therefore, reheats a relatively large volume of ash.
More recently, it has been proposed to incinerate sludge from sewage treatment or water clarification plants in a sludge-pyrolysis process in which the sludge is heated in an oxygen deficient atmosphere to generate a combustible pyrolysis gas. The gas can in turn be used as a heat source for the sludge pyrolysis or it can be otherwise profitly used, e.g. for firing a boiler or driving a gas powered motor.
The system described in the preceding paragraphs has shortcomings such as the fact that it normally requires the input of external energy due to the presence of relatively large amounts of water in the sludge and that the quality of the treated water is no better than that obtained in the sedimentation basin of the plant. Fine particles as well as many dissolved substances remain in the water so that the effluent is at best a water that has a somewhat lesser pollution level than the original waste water fed to the clarification plant. Although it is possible to further treat the water before it is discharged from the plant by means of sand filters and the like, the very high water volume and the difficulty in maintaining the filters unclogged have made such approaches in the past technologically as well as economically unfeasible. To thoroughly clean the water and render it reusable, as by filtering it with activated carbon, has heretofore been impossible because of the prohibitively high costs of activated carbon. Sand filters, on the other hand, represent difficult or impossible cleaning and/or disposal problems, particularly when large quantities of water must be treated as is the case in municipal sewage treatment plants, for example.
Thus, as a result of the above summarized shortcomings in solid and liquid waste disposal systems and techniques, each is performed separately in a generally unsatisfactory manner and the problems connected with each increase continuously while pollution, particularly water pollution becomes more and more serious. Unless the trend is reversed, the day on which widespread water shortages are commonplace is not too far in the distance.